INT-3181: Improve JdbcChannelMS usingIdCache doc

JIRA: https://jira.springsource.org/browse/INT-3181

INT-3181: Fix JdbcChannelMS#pollMessageFromGroup

* Add `doRemoveMessageFromGroup` and check its result.
If it is `false` then just return `null`, not polled `Message`
* Add `AbstractTxTimeoutMessageStoreTests#testInt3181ConcurrentPolling` test
and implement it for some DBs
* Make a note regarding MVCC as important in the Doc

Minor Polishing